The present invention relates to illumination systems, e.g. for extreme ultraviolet radiation. More particularly, the invention relates to the application of such a device in lithographic projection apparatus comprising:
an illumination system constructed and arranged to supply a projection beam of radiation;
a first object table provided with a mask holder constructed to hold a mask;
a second object table provided with a substrate holder constructed to hold a substrate; and
a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection systems, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic Apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatus are described in International Patent Applications WO 98/28665 and WO 98/40791, for example.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die at once, such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In a lithographic apparatus, the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. Whilst most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray and possible sources include laser plasma sources or synchrotron radiation from electron storage rings. An outline design of a lithographic projection apparatus using synchrotron radiation is described in xe2x80x9cSynchrotron radiation sources and condensers for projection x-ray lithographyxe2x80x9d, J. B. Murphy et al, Applied Optics Vol. 32 No. 24 pp 6920-6929 (1993). Although the synchrotron radiation emitted by a storage ring is well confined to the plane containing the circulating electron beam, it is emitted in all directions in that plane, and to generate a sufficiently intense projection beam it is necessary to collect synchrotron radiation from a wide range of angles. This results in an undesirably large device overall and in particular requires the provision of large collection mirrors.
So-called xe2x80x9cundulatorsxe2x80x9d and xe2x80x9cwigglersxe2x80x9d have been proposed as an alternative source of extreme ultraviolet radiation. In these devices, a beam of electrons traveling at high, usually relativistic, speeds, e.g. in a storage ring, is caused to traverse a series of regions in which magnetic fields perpendicular to the beam velocity are established. The directions of the magnetic field in adjacent regions are mutually opposite, so that the electrons follow an undulating path. The transverse accelerations of the electrons following the undulating path causes the emission of Maxwell radiation perpendicular to the direction of the accelerations, i.e. in the direction of the undeviated path. Such radiation sources generally have a moderate or small xc3xa9tendue, as compared to laser plasma sources, for example, which have a large xc3xa9tendue.
The term xe2x80x98xc3xa9tenduexe2x80x99 refers to the product of the size of the source and the solid emission angle.
It is an object of the present invention to provide an optical system that may be used to shape radiation emitted from a radiation source, especially extreme ultraviolet radiation, into an arch- or ring-shaped projection beam for a lithographic projection apparatus.
According to the present invention there is provided lithographic projection apparatus for imaging a mask pattern in a mask onto a substrate, the apparatus comprising:
an illumination system constructed and arranged to supply a projection beam of radiation;
a first object table provided with a mask holder constructed to hold a mask;
a second object table provided with a substrate holder constructed to hold a substrate; and
a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate; characterized in that said illumination system comprises:
scattering means constructed and arranged to control the divergence of said projection beam, said scattering means comprising a one-dimensional array of curved reflecting elements each conforming to a curved surface such as would reflect a narrow and collimated incident beam into a curved fan.
The present invention therefore provides an illumination system which can be used in a lithography apparatus to provide an arch or ring field illumination for the reticle and also good filling of the entrance pupil of the projection system. Furthermore, the invention enables provision to be made for the location of field masks (REMA) and pupil masks (for the control of the filling factor) in the system.
The arch shape of the illumination at the reticle is due to the scattering mirror which is a one-dimensional (at least) array of, for example, toroidal, cylindrical or conical mirrors. For light sources of large xc3xa9tendue, the scattering mirror is preferably a matrix of mirrors, each being a one-dimensional array of toroidal, cylindrical or conical mirrors. The individual mirrors of the matrix can be individually oriented to concentrate the radiation in the projection beam. For light sources with small 6tendue, a second scattering mirror can be introduced into the system to control and improve pupil filling. The second scattering mirror can be a two-dimensional array of aspherical mirrors.
In various embodiments of the invention, relay (or imaging) mirrors can be provided. For example, a relay or collector mirror in front of the first scattering mirror can be provided to collect light from the source and direct it to the first scattering mirror at an appropriate angle of incidence. Relay mirrors behind the first scattering mirror can be provided to produce conjugate planes for field and pupil masking, to direct the light to the reticle and the entrance pupil and to preserve the shape of the arched beam reflected by the first scattering mirror so that the illumination at the reticle has the shape of a ring field.
The second scattering mirror, if provided, may be combined with a relay mirror into a single element. Such a combined element will comprise a two-dimensional array of spherical or aspherical mirrors, effective to perform the scattering function, overlaid on a curved surface, effective to perform the focusing function. The use of fewer elements is advantageous in reducing reflection losses.
The radiation source which provides the projection beam may be an undulator or a wiggler, which emits radiation of a narrow range of wavelengths in a beam with a small divergence angle, or a laser plasma source, which emits radiation into a wider range. The simple collection optics of the invention, enable the provision of a powerful and well-shaped projection beam for a lithographic projection apparatus.
The present invention also provides a device manufacturing method using a lithography apparatus, the method comprising the steps of:
providing a substrate which is at least partially covered by a layer of energy-sensitive material;
providing a mask containing a pattern;
using a projection beam of radiation to project an image of at least part of the mask pattern onto a target area of the layer of energy-sensitive material; characterized in that:
the divergence of said projection beam is controlled in an illumination system of the lithography apparatus using scattering means comprising a one-dimensional array of curved reflecting elements each conforming to a curved surface such as would reflect a narrow and collimated incident beam into a curved fan.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallisation, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing. The individual devices can then be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.